The present invention relates to an electric conductor including a conductive path composed of conductor or semiconductor fine particles and organic semiconductor molecules, a method for producing the same, a semiconductor device using the electric conductor, and a method for producing the semiconductor device.
Thin film transistors (abbreviated to “TFTS” hereinafter) are widely used as switching elements in electronic circuits, particularly active matrix circuits of displays or the like.
At present, most of the TFTs are Si-based inorganic transistors each using amorphous silicon (a-Si) or polycrystalline silicon (poly-Si) for a semiconductor layer (channel layer). These transistors are produced by a process using plasma CVD (Chemical Vapor Deposition) for forming a semiconductor layer, and thus the process is a high cost process. In addition, heat treatment is performed at a high temperature of about 350° C., thereby increasing the process cost and causing limitation of a substrate.
In recent years, organic semiconductor transistors using organic semiconductor materials have been actively developed because these transistors can be produced by a low cost process such as spin coating or dipping at a low temperature, and also a film can be formed on a flexible substrate with no heat resistance, such as a plastic substrate.
However, only a value of 10−3 to 1 cm2/Vs has been attained as mobility, which is a characteristic index of TFTs, by using organic semiconductor materials (C. D. Dimitrakopoulos et al., Adv. Mater., 14, 99 (2002)). This value is lower than several cm2/Vs of a-Si and about 100 cm2/Vs of poly-Si, and does not reach a mobility of 1 to 3 cm2/Vs for display TFTs. Therefore, an improvement of mobility becomes a great problem in developing organic semiconductor materials.
The mobility of an organic semiconductor material is determined by intramolecular charge transfer and intermolecular charge transfer. The intramolecular charge transfer can be produced by a conjugated system formed due to delocalization of π electrons. The intermolecular charge transfer is produced by conduction due to intermolecular bonding, conduction due to molecular orbital overlap by Vander Waals force, or hopping conduction through an intermolecular trapping level.
In this case, when intramolecular mobility is denoted by “μ-intra”, mobility due to intermolecular bonding is denoted by “μ-inter”, and mobility due to intermolecular hopping conduction is denoted by “μ-hop”, the following relation is established:
μ-intra≧μ-inter>μ-hop
In an organic semiconductor material, the entire mobility is limited by slow charge transfer between molecules, thereby decreasing charge mobility.
For example, when a thin film of an organic semiconductor material, e.g., pentacene, is formed by vapor deposition, in order to improve mobility, the rate of vapor deposition is minimized, and the substrate temperature is suppressed to room temperature to improve molecular orientation, thereby achieving a mobility of 0.6 cm2/Vs (C. D. Dimitrakopoulos et al., IBM. J. Res. & Dev., 45, 11 (2001)).
This is intended to improve mobility by improving the crystallinity of the material and suppressing intermolecular hopping conduction. Although the mobility is slightly improved, the entire mobility is still limited by intermolecular mobility, thereby causing difficulty in achieving satisfactory mobility.
Other attempts have been made to improve electric properties by combining an organic semiconductor material with another material.
For example, Japanese Unexamined Patent Application Publication No. 2003-301116 discloses an example in which an organic semiconductor material includes a mixture of a conjugated polymer and an organometallic complex. The conjugated polymer is a polymer in which bonds are conjugated in respective repeat units, which constitute a polymer main chain, and conjugated between the respective repeat units. However, in the mixture, a chemical bond is not formed between the conjugated polymer and the organometallic complex, and thus conductivity is not so improved.
Also, Japanese Unexamined Patent Application Publication No. 2004-6827 discloses an example in which a conductive region including a conductive material is provided in an organic semiconductor layer while avoiding short-circuit between a source electrode and a drain electrode. The conductive region is provided for shortening the effective channel length, not for improving the electric characteristics of the organic semiconductor material, such as mobility, and a chemical bond is not formed between the organic semiconductor and the conductive material.
Furthermore, Japanese Unexamined Patent Application Publication No. 2004-88090 discloses a semiconductor device including conductor or semiconductor fine particles and organic semiconductor molecules bonded to the fine particles to form a network-type conductive path so that the conductivity of the conductive path can be controlled by an electric field, and a method for producing the semiconductor device.
FIG. 9A is a sectional view of the insulated-gate field-effect transistor disclosed in Japanese Unexamined Patent Application Publication No. 2004-88090, and FIG. 9B is an enlarged view of the principal portion thereof. In the field-effect transistor, fine particles 109 of gold or the like are bonded in a network form to organic semiconductor molecules 112 of 4,4′-diphenyldithiol or the like to form a channel layer 108 between a source electrode 104 and a drain electrode 105, so that the carrier transfer in the network aggregate is controlled by the gate voltage applied to a gate electrode 102.
As shown in FIG. 9B, in the aggregate, the organic semiconductor molecules 112 are bonded to the fine particles 109 through functional groups at both ends of each of organic semiconductor molecules 112 so that the fine particles 109 and the organic semiconductor molecules 112 are alternately connected to form a conductive path in which the conductive paths in the fine particles 109 and the conductive paths in the organic semiconductor molecules 112 are connected together. Since a plurality of organic semiconductor molecules 112 can bond to each fine particle 109, a two-dimensional or three-dimensional network conductive path is formed as a whole.
The conductive path does not include intermolecular electron transfer, which causes low mobility in a related-art organic semiconductor, and electron transfer in an organic semiconductor molecule is produced through a conjugated system formed along the molecular skeleton. Therefore, high mobility is expected.
FIGS. 10A to 11G show a flow chart of a process for manufacturing the insulated-gate field-effect transistor shown in FIGS. 9A and 9B. Description will be made on the assumption that the fine particles 109 are gold fine particles, and the organic semiconductor molecules 112 are molecules of 4,4′-biphenyldithiol.
Step 1
First, as shown in FIG. 10A, the gate electrode 102, the gate insulating film 103, the source electrode 104, and the drain electrode 105 are formed on a substrate 101 such as a plastic substrate. For example, the electrodes 102, 104, and 105 are formed by vapor deposition of gold, and the gate insulating film 103 is formed by spin-coating a polymethyl methacrylate (PMMA) solution and then evaporating the solvent.
Step 2
Next, a surface of a region in which the channel layer 108 is to be formed is immersed in, for example, a toluene or hexane solution of 3-aminopropyltrimethoxysilane (APTMS) used as solder molecules 107 and washed with the solvent to replace the solution with the solvent, and then the solvent is evaporated to form a molecular solder layer 106 as an underlying layer for fixing only one layer of the gold fine particles 109, as shown in FIG. 10B. APTMS can bond to the gate insulating film 103 through a silanol group at one of the ends, and also can bond to each gold fine particle 109 through an amino group at the other end. Therefore, each of the solder molecules 107 is a molecule which can bond to the gate insulating film 103 at one of the ends and also bond to each fine particle 109 at the other end, and thus has the function to fix each fine particle 109 to the gate insulating film 103.
Step 3
Next, the substrate 101 is immersed in a dispersion (concentration: several mM) of the gold fine particles 109 in a solvent such as toluene, chloroform, or the like for several minutes to several hours, and then the solvent is evaporated. As a result, as shown in FIG. 10C, the gold fine particles 109 are fixed to the surface of the molecular solder layer 106 on the substrate 101 to form a gold fine particle layer 109a including the gold fine particles 109 on the molecular solder layer 106. In this step, only one gold fine particle layer 109a is fixed to the molecular solder layer 106 through the amino groups. Excessive gold fine particles 109 not fixed to the molecular solder layer 106 are washed out.
The gold fine particles 109 are colloidal particles having a particle size of 10 nm or less. In order to stably disperse the gold fine particles 109 in the solvent such as toluene, chloroform, or the like, protective film molecules are adhered to the fine particles 109 to coat each particle 109 with a protective film 110, for preventing aggregation and precipitation of the fine particles 109. The gold fine particles 109 each coated with the protective film 10 are fixed to the molecular solder layer 106. The solder molecules 107 bond to the gold fine particles 109 by substituting some of the protective film molecules. However, as shown in FIG. 10C, most of the protective film molecules remain to bond to the gold fine particles 109.
Step 4
Then, the substrate 101 is immersed in a toluene solution (concentration: several mM or less) of 4,4′-biphenyldithiol and washed with the solvent to replace the solution with the solvent, and then the solvent is evaporated. In this step, as shown in FIG. 10D, the 4,4′-biphenyldithol molecules 112 react with the gold fine particles 109 through the terminal thiol groups —SH of the molecules to substitute the protective film molecules which form the protective films 110, thereby bonding to the surfaces of the gold fine particles 109. In this case, a plurality of 4,4′-biphenyldithiol molecules 112 bond to the surface of each gold fine particle 109 so as to surround the particle 109. Since some of the 4,4′-biphenyldithiol molecules 112 bonding to one of the particles 109 bond to other ones of the gold fine particles 109 through the thiol group at the other end of each of the molecules to form a first channel layer 108a in which the gold fine particles 109 are connected in a two-dimensional network through the 4,4′-diphenyldithiol molecules 112.
Since many unreacted thiol groups of the 4,4′-biphenyldithiol molecules 112 remain on the surface of the channel layer 108a, the surface of the channel layer 108a has strong bonding force to the gold fine particles 112.
Step 5
Next, as shown in FIG. 11E, the substrate 101 is immersed in a dispersion of the gold fine particles 109 in a solvent such as toluene, chloroform, or the like for several minutes to several hours, and then the solvent is evaporated as in step 3. As a result, the gold fine particles 109 are fixed and bonded to the surface of the first channel layer 108a to form a second gold fine particle layer 109b. In this step, the gold fine particles 109 in the second layer are connected to the gold fine particles 109 in the first layer through the 4,4′-biphenyldithiol 112. Also, the fine particles 109 in the first layer which are connected to the same gold fine particle 109 in the second layer are connected together indirectly through the same gold fine particle 109 in the second layer, so that the gold fine particles 109 are connected together in a three-dimensional form. Excessive gold fine particles 109 not fixed to the channel layer 108a are washed out.
Like in Step 3, in order to prevent aggregation of the gold fine particles 109, the gold fine particles 109 are each coated with the protective film 110 and fixed to the channel layer 108a. Although unreacted thiol groups of the 4,4′-biphenyldithiol molecules 112 remaining on the surface of the channel layer 108a bond to the gold fine particles 109 by substitution of the protective film molecules, most of the protective film molecules remain to bond to the gold fine particles 109, as shown in FIG. 11E.
Step 6
Then, like in Step 4, the substrate 101 is immersed in a solution of the 4,4′-diphenyldithiol 112 in toluene at a concentration of several mM or less and washed with the solvent to replace the solution with the solvent, and then the solvent is evaporated. As a result, as shown in FIG. 11F, many 4,4′-biphenyldithiol molecules 112 bond to each gold fine particle 109 so as to envelope the particle 109, thereby forming a second channel layer 108b in which the gold fine particles 109 are connected to each other through the 4,4′-biphenyldithiol molecules 112.
Thereafter, steps 5 and 6 are repeated several times to form a channel layer 108 each time in which a three-dimensional network conductive path is formed, as shown in FIG. 11G. The channel layer 108 having a desired thickness can be formed by appropriately selecting the number of repeats. The above-described method for forming the gold fine particle layer is referred to M. D. Musick et al., Chem. Mater., 9, 1499 (1997) and Chem. Mater., 12, 2869 (2000).
The conductive path disclosed in Japanese Unexamined Patent Application Publication No. 2004-88090 does not include intermolecular electron transfer, and thus mobility is not limited by the intermolecular electron transfer. Therefore, the mobility of the conductive path (in the axial direction of a molecular) along the main chain of an organic semiconductor molecule, for example, high intramolecular mobility due to nonlocalized π electrons, can be maximized.
However, in order to prevent aggregation and precipitation of the fine particles 109 of gold or the like in the process for forming the conductive path, the fine particles 109 are coated with the protective film molecules for preventing aggregation in formation of a colloidal solution of the fine particles. In the method for forming a semiconductor device disclosed in Japanese Unexamined Patent Application Publication No. 2004-88090, therefore, a dispersion of the fine particles 109 each coated with the protective film 110 in the solvent is applied to the substrate to form the fine particle layers 109a and 109b and fix the fine particles 109 to the substrate, as shown in FIGS. 10C and 11E. Then, as shown in FIG. 10D and 11F, the organic semiconductor molecules 112 each having functional groups at both ends, which can strongly bond to the fine particles 109, are reacted to substitute the protective film molecules with the organic semiconductor molecules 112 and to connect the fine particles 109 to each other through the organic semiconductor molecules 112, thereby forming a network conductive path including the final particles 109 and the organic semiconductor molecules 112.
In order to achieve high efficiency-networking of fine particles which influences the performance of a semiconductor device, there are the following conditions:
(1) In order to increase the ratio of linkage between the fine particles 109 through the organic semiconductor molecules 112 which substitute the protective film molecules, the distance between the fine particles 109 in each of the fine particle layers 109a and 109b is controlled to at least the maximum length or less of the organic semiconductor molecules 112 and preferably a length desirable for linkage through the organic semiconductor molecules 112, for example, about the natural length of the organic semiconductor molecules 112.
(2) In order to effectively promote substitution reaction, the protective film molecules used have smaller bonding force to the fine particles 109 than that to the organic semiconductor molecules 112.
However, it is difficult to precisely control the distance between the fine particles in each of the fine particle layers 109a and 109b. Also, when the bonding force of the protective film molecules to the fine particles 109 is excessively small, the function to protect the fine particles 109 becomes insufficient. Therefore, in order to achieve the sufficient protective function and satisfy the above-described condition (2), the bonding ability of the protective film molecules for the fine particles 109 is greatly limited, thereby causing difficulty in finding appropriate protective film molecules. As a result, the method disclosed in Japanese Unexamined Patent Application Publication No. 2004-88090 has difficulty in achieving high-efficiency networking of fine particles.